1. Field of the Invention
The present invention generally relates to electronic devices, such as thin film transistors (TFTs) and flat panel display devices including the same, and more particularly, to an electronic device, and flat panel display device including the same, in which electrostatic damage caused by static electricity is prevented or reduced.
2. Description of the Related Art
Many kinds of display devices are used for displaying images. Recently, a variety of flat panel display devices have replaced cathode ray tubes (CRTs). Flat panel display devices may be classified as either emissive or non-emissive, depending on the type of light emission used. Emissive display devices include CRTs, plasma display panel devices, vacuum fluorescent display devices, field emission display devices, and organic/inorganic electro-luminescent display devices, and non-emissive display devices include liquid crystal display devices. Flat panel emissive organic electroluminescent display (OELD) devices draw attention since they do not include a light emitting device, such as a back light, and are capable of operating with low power consumption and at high efficiency. Advantages of OELD devices include low operating voltage, a light weight, a thin profile, wide viewing angles, and fast video response times.
A conventional electroluminescent unit of an OELD device includes a first electrode (anode) that is formed in a stack on a substrate, a second electrode (cathode), and an organic light-emitting layer (thin film) interposed between the first and second electrodes. In operation, OELD devices emit light of a specific wavelength using energy generated from exitons formed from recombining electrons injected from the anode and holes injected from the cathode into the organic thin film. To increase the efficiency of light emission, an electron transport layer (ETL) may be interposed between the cathode and the organic emitting layer. Similarly, a hole transport layer (HTL) may be interposed between the anode and the organic emitting layer. Also, a hole injection layer (HIL) may be disposed between the anode and the HTL and an electron injection layer (EIL) may be interposed between the cathode and the ETL.
A passive matrix organic electro-luminescent display (OELD) device uses a manual driving method, while an active matrix (AM) OELD device uses an active driving method. In the passive matrix OELD device, the anodes are arranged in columns and the electrodes are arranged in rows. Scanning signals are supplied to the cathodes from a row driving circuit, and data signals are supplied to each pixel from a column driving circuit. On the other hand, the active matrix OELD device controls a signal inputted to a pixel using a thin film transistor (TFT) and is widely used for implementing animation since it is suitable for processing a large number of signals virtually simultaneously.
A disadvantage associated with conventional active matrix OELD devices is that one or more faulty pixels may develop in the devices' display regions due to static electricity generated when manufacturing, or operating, the OELD devices. Examples of correctly functioning and faulty pixels are shown in FIG. 1A, FIG. 1B, and FIG. 1C.
FIG. 1A is a plan view photograph of a conventional OELD device that shows faulty pixels as bright spots. FIG. 1B is a magnified photograph of a normal pixel indicated as A in FIG. 1A, and FIG. 1C is a magnified photograph of faulty pixel indicated as B in FIG. 1A. FIGS. 1B and 1C are bottom views of the conventional OELD device of FIG. 1A. These bottom views are taken from the OELD's substrate side looking through the multilayered structure of the substrate and the various electrical and electroluminescent components formed on it. Thus, in FIGS. 1B and 1C, the gate lines 3a and 3b appear to be positioned above the conductive layer 5.
In FIGS. 1B and 1C, each of the pixels 1a and 1b comprises an electroluminescent unit, a gate electrode (2a in FIG. 1A and 2b in FIG. 1B), and a light emitting thin film transistor (Ma in FIG. 1B and Mb in FIG. 1C) that transmits electrical signals from a driving thin film transistor (not shown) to the pixel. Source electrodes of the light emitting thin film transistors Ma and Mb are electrically connected to the driving thin film transistors (not shown) via conductive layers 5.
FIG. 1D is a magnified plan view of a portion indicated as B′ in FIG. 1C. Referring to FIG. 1D, a conductive layer 5 may extend across other conductive layers. In the magnified bottom view of FIG. ID, for example, the conductive layer 5 is shown crossing the gate line 3b. In this exemplary drawing, the gate line 3b appears to be positioned above the conductive layer 5. In operation, the gate line 3b may act as a scan line and/or an extension unit of a scan line for supplying electrical signals to a thin film transistor.
To meet design specifications, the width of each gate line 3b may change along a length thereof. In the conventional design illustrated in FIGS. 1B, 1C, and 1D, for example, each gate line 3b changes in width at a portion thereof that crosses the conductive layer 5. As shown in FIG. 1D, the wider portion of the gate line 3b may be a width change part Aw, and a narrower connected portion of the gate line 3b may be a crossing unit Ac. Both the width change part Aw and the crossing unit Ac are positioned above the conductive layer 5 and within the side bounds thereof. During manufacture of the conductive layers, the conductive layer 5 may accumulate an electrostatic charge. Because electricity tends to discharge at pointed regions of a conductor, an electrostatic discharge (ESD) tends to occur at angled portions Ad of the width change part Aw shown in FIG. 1D. In most cases, the ESD damages the corresponding pixel 1a/1b, causing it to overluminate (e.g, appear as a bright spot, such as the bright spot B shown in FIG. 1A). Such an electrostatic discharge is easily induced since static electricity is concentrated at the crossing portion, and thus the possibility of generating a short circuit between conductive layers increases if an insulating layer interposed between the conductive layers is damaged. As depicted in FIGS. 1B and 1C, even though the same desired electrical signal is inputted to the pixel la in FIG. 1B and the pixel 1b in FIG. 1C, the pixel 1b in FIG. 1C malfunctions and produces a bright spot having a greater brightness than the normal pixel la in FIG. 1B. The greater brightness occurs because the short circuit between different conductive layers 3b and 5 creates and applies a different electrical signal than one that is desired. This undesired electrostatic discharge may seriously degrade a flat panel OELD device's picture quality, which requires high uniformity over an entire display region of the OELD.